I think you've gotten some great advice, so far.

I don't know which vids and online docs you were using, but ... I'd recommend starting with the Park Tool site -- particularly their pages about adjusting the front and rear derailleurs. You have to do these things step by step, in the order in which Park presents them.

But ... yeah ... a thorough cleaning and lubrication comes first. You might even want to replace all the cables before you start the adjustment. While I don't see anything obviously wrong with the bike, it does suffer from neglect

Be patient. Be thoughtful. Take your time. As you move through each step, try to ask yourself what that step means, and how the adjustment you're doing will impact the system that is your bike. The more you can get your head around the function of each item, the better your future efforts will be.

THEN, you will be a TRUE Shaolin master